The present invention relates to a short carbon fiber bundling mass mainly used for producing a fiber-reinforced resin composition, a process for producing the short carbon fiber bundling mass, and the fiber-reinforced resin composition containing the short carbon fiber bundling mass. More particularly, the present invention relates to a fiber-reinforced resin composition containing a short carbon fiber bundling mass, which is excellent in rigidity, heat resistance, lightweight ability and moldability, and therefore, is useful as a material in place of a metal diecast material composed of aluminum or the like.
Precision parts have been hitherto produced from metal using a die-casting method in view of a bending modulus, a dimensional accuracy, a heat resistance or the like. Such metal diecast products can be finished with less machining as compared with normal shaved products. However, the diecast products still require considerable machining tasks to form a final product. Since a die used in the die-casting method is exposed to an elevated temperature, it must be formed from an expensive heat-resistant material. In addition, since the diecast method includes a step of melting a metal material, an additional cost is required to install surrounding refractory facilities. Furthermore, it is required to lighten a weight of precision machinery used in the diecast method.
In recent years, particular a fiber-reinforced resin composition produced by mixing and dispersing a carbon fiber in various matrix resins are attracting attention as an industrially important material because of its excellent mechanical properties such as a high mechanical strength, a high rigidity, a low specific gravity, a high impact resistance or the like. Especially, in the fields of the precision parts and electronic parts which have been recently further miniaturized and lightened, the molded products composed of a rigid fiber-reinforced resin composition have been used as the substitute of the metal diecast products. However, the conventional fiber-reinforced resin compositions are still inferior in rigidity and heat resistance as compared with the metal diecast material, resulting in limiting the use of the fiber-reinforced resin composition. For example, though the fiber-reinforced resin composition has a heat resistance capable of withstanding a temperature of about 200.degree. C., a thermal conductivity thereof is still low, thereby causing such disadvantage that high-temperature portions are locally generated due to deteriorated heat flow in the molded product upon use. For this reason, the molded product prepared therefrom cannot yet be sufficiently put into a practical use.
Conventionally, when the fiber-reinforced resin composition is produced by blending and dispersing short carbon fibers in various matrix resins, a number of long carbon fibers are first bundled by using a so-called sizing agent and then the bundled long carbon fibers are cut into short carbon fiber bundling masses, followed by mixing the short carbon fiber bundling masses in the matrix resin to obtain a fiber-reinforced resin composition.
In the case where the fiber-reinforced resin composition is prepared by blending the short carbon fiber bundling mass produced by using the sizing agent, with a matrix resin, especially a matrix resin used under a high temperature condition, the carbon fiber-containing resin composition is subjected to a high-temperature molding process to form a molded product, upon which the resin composition is likely to generate a gas. Such gas generation causes poor environmental conditions. In order to solve the problem, the present inventors have previously proposed, in Japanese Patent Application Laid-open (KOKAI) No. 6-143,483 (1994), a fiber-reinforced resin composition containing a short carbon fiber bundling mass prepared by carbonizing the short carbon fiber mass bundled by using the sizing agent at a temperature of 400 to 1,500.degree. C. In this Japanese Patent KOKAI, there has been no suggestion that, the short carbon fiber bundling mass is treated at a higher temperature than the afore-mentioned temperature range of 400 to 1,500.degree. C.
On the other hand, in the field of such a fiber-reinforced resin composition, in association with an increasing demand for miniaturization and light-weight of the molded product, it is required that a blending ratio of the carbon fiber to the matrix resin is increased as highly as possible to obtain a high-rigidity fiber-reinforced resin material.
However, when a large amount of the conventional short carbon fiber bundling mass or bunched short carbon fiber mass which has been subjected to carbonization treatment, for example, not less than 30 parts by weight of the short carbon fiber bundling mass or bunched short carbon fiber mass are mixed with and dispersed in 100 parts by weight of the matrix resin, various problems are caused so that it is difficult to attain the resin composition having a high carbon fiber content, which become difficult to obtain a fiber-reinforced resin composition having a high rigidity and a high bending modulus because of insufficient dispersion of the carbon fiber in the matrix resin. For instance, when a large amount of the short carbon fiber bundling mass is supplied into a feeder, there occur such a problem that pills or flocks are generated so that clogging is caused in a feeding passage, or it is not possible to uniformly disperse the short carbon fiber in the matrix resin. In addition, it is difficult to obtain a short carbon fiber bundling mass having a high bulk density even though high-modulus carbon fibers are bundled by using a sizing agent.
In the fields where a molded product of a miniaturization and light-weight is required, it is demanded to provide a high-modulus fiber-reinforced resin composition. However, conventionally, even if a large amount of the short carbon fiber bundling mass is incorporated into the matrix resin, it is possible to obtain the molded product exhibiting a high bending modulus at a required level.
Further, it is demanded to provide a short carbon fiber bundling mass having a fiber length of 2 to 20 mm and a bulk density of 450 to 650 g/l.
As a result of the present inventor intense studies to solve the afore-mentioned problems, it has been found that by subjecting bundled short carbon fiber formed by using a sizing agent, to graphitization in an inert gas atmosphere at a maximum temperature ranging from 1600.degree. C. to 3300.degree. C., the obtained short carbon fiber bundling mass composed of short carbon fiber and graphite derived from the resin as the sizing agent, shows a good handing ability in mixing and dispersing processes and a high dispersibility when mixed with a resin material, so that it is possible to incorporate a large amount of carbon fiber in a matrix resin and a resultant resin composition unexpectedly exhibit an extremely high bending modulus and thermal conductivity. The present invention has been attained on the basis of the finding.